HARRISBURG, Pa. (AP) — Federal transportation safety officials told Philadelphia's mass transit agency this week that it should shelve an aging electric railcar model that is heavily used in its regional rail fleet until it figures out how to stop them from catching fire. The recommendation from the National Transportation Safety Board came after it investigated five fires this year involving the Silverliner IV passenger railcars used by the Southeastern Pennsylvania Transportation Authority, or SEPTA. One railcar was involved in two of the fires, and two other railcars were destroyed, the NTSB said. SEPTA is one of the nation's largest mass transit agencies, carrying 800,000 daily riders on buses, trolleys and rail. In its budget report issued earlier this year, SEPTA reported that ballooning material, manufacturing and construction costs has made it more expensive for it to replace the Silverliner IV fleet.